Virtually all electronic devices require some type of power converter to assure that electronic circuits therein receive power at the appropriate voltage even when there is wide variation in the current drawn by the load that such circuits constitute. Many commercially available integrated circuits adopt peak current mode (PCM) control to supply power to the switching circuits therein. However, peak current mode control only controls the peak current but cannot provide accurate average current control. Further, peak current mode control suffers from sub-harmonic instability and an external ramp signal is required to stabilize operation. Providing an appropriate external ramp signal not only complicates the systems but substantially slows the loop response due to overcompensation to provide stability under worst-case input/output voltage and current conditions and variations in inductance value due to design preference, temperature variation and DC current bias. In contrast, average current mode (ACM) control is often used for various applications such as multi-phase voltage regulators (VRs), point of load (POL) converters, light-emitting diode (LED) drivers, battery chargers, power factor correctors and the like since they generally provide very precise current control, good current sharing in multi-phase converters and accurate over-current protection.
However, average current mode control for switching power converters presents two significant performance issues that tend to limit their suitability for some applications. First, ACM has potential sub-harmonic stability issues. Second, transient response is relatively slow while current commands for LED drivers, for example, can be large and rapid and, in applications where the required output current is transiently changed, average current mode control node can require a substantial number of switching cycles to settle at the new current value. Slow current transient response also leads to slow output voltage transient response in applications where fast, dynamic voltage response is required such as in voltage regulators for microprocessors. Third, since average current mode control is usually performed at a near constant switching frequency, the light load efficiency drops dramatically due to switching losses and driving losses when reduced current is supplied to the load.
Several approaches have been attempted to improve transient response by altering the structure of the current feedback loop but none have been entirely successful and none have addressed the issue of low light load efficiency even at the cost of substantial added complexity. Further, some approaches to improving transient response have introduced other limitations on applicability such as requiring operation in a continuous current mode (CCM) and being inoperable for discontinuous current mode (DCM) operation. These problems with known approaches to improving transient response will be discussed in greater detail below.